STAT3 INHIBITION IS A SYNTHETIC LETHAL VULNERABILITY FOR SF3B1-MUTATED HEMATOPOIETIC STEM CELLS

Hematopoietic stem and progenitor cells (HSPCs) maintain hematopoiesis throughout the lifetime of an organism. HSPC dysfunction can lead to disorders such as myelodysplastic syndrome (MDS). Spliceosome mutations are prevalent in MDS, but how splicing regulates HSPCs remains poorly understood. Using a zebrafish loss-of-function mutant for the top mutated splicing factor in MDS, splicing factor 3b, subunit 1 (sf3b1), we found that impaired splicing hindered HSPC specification. To identify Sf3b1-regulated transcripts, we performed RNA-sequencing on kdrl:gfp+ endothelial cells from sf3b1 mutant and unaffected siblings at 24hpf. ∼900 genes were mis-spliced with significant enrichment for Signaling Transducer and Activator of Transcription 3 (Stat3) pathway components. Mis-spliced Stat3 signaling components observed in sf3b1 mutants were predicted to dampen protein levels or inhibit functionality, and Stat3 target genes were significantly downregulated in sf3b1 mutants. Constitutively-active Stat3 overexpression in sf3b1 mutants partially suppressed the HSPC defect. Induced mis-splicing of stat3 using morpholinos caused a significant HSPC decrease in sf3b1 heterozygotes at 28hpf, but elicited no effect in wildtype embryos, demonstrating a synthetic lethal interaction. Inhibiting Stat3 with small molecule STATTIC similarly reduced HSPCs in heterozygous but not wildtype embryos. We next tested if STAT3 inhibition selectively affected SF3B1-mutated human cells. K562 cells expressing the MDS-associated SF3B1-K666N mutation were more sensitive to STAT3 inhibition with STATTIC than wildtype cells. Similarly, primary SF3B1-mutant MDS patient cells produced fewer hematopoietic colonies when treated with STATTIC than controls. Together, STAT3 inhibition is a conserved synthetic lethal vulnerability for SF3B1-mutated cells with potential to treat splicing factor-mutated MDS.